Nowadays, by virtue of their superior magnetic properties, rare earth sintered magnets, typically neodymium-based magnets are widely used in motors, sensors and other devices to be mounted in hard disks, air conditioners, hybrid vehicles, and the like.
In general, rare earth sintered magnets are prepared by powder metallurgy as follows. First, raw materials are mixed in accord with a predetermined composition. Using a high-frequency induction furnace, the mixture is melted and cast into an alloy. The alloy is coarsely crushed by a grinding machine such as a jaw crusher, Brown mill or pin mill or hydrogen decrepitation (or hydrogen embrittlement treatment) and then finely milled by a jet mill or the like, obtaining a fine powder having an average particle size of 1 to 10 μm. The fine powder is molded into a compact of desired shape while applying a magnetic field for imparting magnetic anisotropy. The compact is sintered and heat treated to form a sintered magnet.
In the preparation of rare earth sintered magnets by powder metallurgy, the step of molding under a magnetic field typically uses a mold consisting of a die, an upper punch and a lower punch. Molding is carried out by filling the mold cavity defined between the die and the lower punch with the fine powder, and forcing the upper punch to apply a uniaxial pressure to the powder. The mold cavity is fully filled with the fine powder so that the upper surface of the powder fill may be flush with the top of the die.
In the molding step, it is practiced for the purpose of improving the production yield to compression mold the powder fill into a compact shape which is close to the shape of the final magnet product. In an example where the final magnet product is of C shape, the powder is molded into a compact of an approximate C shape. To this end, the pressure surfaces of the upper and lower punches are shaped non-planar. In this case, if the mold cavity is fully filled with fine powder so that the upper surface of powder may be flush with the top of the die, the amount of powder fill in the cavity per height of a magnet product to be molded is non-uniform among horizontally spaced apart positions. When the powder fill is compression molded in this state, the molded compact has a varying density owing to the difference of fill amount. A problem arises when this compact is sintered. Namely, due to a difference in shrinkage between different sites in the compact, the sintered body can be warped or deformed and at the worst, cracked or fissured. These problems invite a drop of production yield.
As means for preventing the sintered body from cracking or fissure, Patent Document 1 discloses a method of chamfering the working surface of a punch, and adjusting the chamfer width and/or refining the roughness of the working surface. Although the method is effective for preventing the sintered body from cracking or fissure, the method is limited to the preparation of magnets of a special shape that permits a mold to be chamfered. Since the problem of compact density pointed out above remains unsolved, the method is substantially ineffective for suppressing the sintered body from warp or deformation.
Patent Document 2 discloses a powder feeder box including a box housing and a guide for leveling the powder flat wherein the powder is smoothed out conformal to the upper shape of the compact to be molded. This method eliminates the difference of fill amount and hence, the variation of compact density. However, the assembly of the feeder box is cumbersome, indicating inefficiency. A number of guides are necessary to meet the shape of every upper punch. The apparatus is thus redundant.